1. Field of the Invention
This invention relates to a circuit for driving inductive loads and more particularly to a monolithic integrated circuit for supplying in rush and sustaining current level drive signals to operate an inductive solenoid device.
2. Description of the Prior Art
Solenoids operated by generating a magnetic field are generally known in the art. The magnetic field is produced by causing current to flow through an inductive field coil. In most applications utilizing solenoids it is desired to initially place a maximum voltage across the field coil to allow the current to ramp therethrough to a peak magnitude, the value of which is limited only by the time constant of the coil. For example, electronic fuel injection systems have injector valves which are comprised of solenoids with inductive coils for opening and closing the valves in timed relationship to the operation of the automotive engine. Hence, the correct cylinder of the engine can be supplied with the correct fuel mixture at the correct sequence in the engine firing cycle. Because of the magnetic characteristics of the inductive field coils whether used in fuel injection systems for driving the magnetic valves or electronic ignition systems wherein once the magnetic field has been established by the current ramping to the peak magnitude, maintenance of the field requires less current. Thus, in order to open the valves as quickly as possible, rapid current buildup in the coil is needed. However, after the valve is initially opened, a reduced or sustaining level of current can then be applied to maintain the valve in an open state. This sustaining current level allows for fast closing of the valves in the correct time relationship and also helps to prevent overheating of the valves which could otherwise occur with the higher level current being maintained for a period of time.
Most, if not all of the prior art, including fuel injection systems, comprises a driver circuit and a discrete power device. The discrete power device is coupled in series between the injector valve, for instance, and a discrete sense resistor. In operation the driver circuit which may be a monolithic integrated circuit, is driven by an input signal in timed relation to the engine operation to turn on the discrete power device at the correct sequence in the engine cycle to allow maximum in rush current to open the valves. Once the peak current level is reached, a voltage developed across the sense resistor is usually compared with a referenced voltage set in the driver circuit to reduce the drive to the discrete power device in order to reduce the field current through the inductive coil to a sustaining level.
In order to reduce component cost for the inductive driver systems it is desirable to have the driver circuit, the power device and sense resistor on one single monolithic integrated circuit chip.